This invention relates generally to single-stage power converters, and more particularly to single-stage power converters having an AC, a DC, or an AC/DC input and providing power factor correction.
As the number of computers and sophisticated electronics continues to grow, so to does the requirement for high-quality electric power to supply the growing demand presented by these devices. Unfortunately, the ability of electric power utilities to supply the power quality required by such equipment at the point of utilization has simply not kept up with the need for such power. As a result, there is a greatly increased need for power conversion and conditioning equipment that is capable of supplying reliable, high-quality power for use by these devices. Additionally, despite the essential role that such power conversion and conditioning equipment plays, there is increased market pressure for such equipment to be as low-cost and complexity as possible to achieve the required reliability demanded by the consuming public.
Uninterruptible power supplies (UPSs) make up one class of such power conversion and conditioning equipment in increased demand. These UPSs are needed to supply reliable high-quality power to the consumer and commercial electronics industries to maintain operation of this equipment in the face of degraded or absent utility power. A typical UPS includes power conversion circuitry capable of conditioning poor quality AC voltage from the utility line, as well as generating high-quality output power from electric storage batteries. The typical UPS utilizes a multistage power converter to fulfill all of the requirements of the UPS system. These requirements may include maintaining the charge of the electric storage batteries during AC power line usage, performing power factor correction on the input power draw from the utility line, and generating an AC output voltage from the electric storage batteries during periods of loss or sufficient degradation of the power quality of the utility line voltage. Unfortunately, such increased complexity also tends to drive up the cost of such UPS equipment.
Various single-stage power converter topologies are known in the power conversion industry. Unfortunately, most are limited to simple DC-to-DC conversion. One such single-stage power converter, which has found wide application as a DC-to-DC converter, is known as the Ćuk converter after its inventor, Dr. Slobodan Ćuk of the California Institute of Technology. This single-stage switched mode DC-to-DC converter 400 illustrated in FIG. 4 operates from a DC input voltage source 402 and delivers an output DC voltage to the connected load 416. The DC source 402 is coupled through input inductor 404 and coupled across switching device 406. The converter 400 includes capacitor 408 and diode 410, as well as an output inductor 412 and filter capacitor 414. The control and operation of the Ćuk converter 400 is well known, and has an output transfer function defined by the following equation:             V      OUT        =                  -                  V          IN                    xc3x97                        T          ON                          T          OFF                      ,
where Ton and Toff are the on and off times of the switching element 406.
As will be recognized by those skilled in the art, this converter may be operated in a boost or buck fashion. As will also be recognized, the Ćuk converter 400 is inverting, that is the output voltage is of opposite polarity to the input voltage. It is of interest to note that the Ćuk converter 400 allows for a continuous input current and a continuous output current. However, limitations on this circuit require that the input voltage be equal to or greater than zero, thereby constraining its operation to applications having DC input voltages only. As such, the Ćuk converter 400 has not found applicability where AC input voltage is used.
Another switched mode single-stage power converter that has found wide applicability is the single ended primary induction converter (SEPIC) 500 as illustrated in FIG. 5. As with the Ćuk converter 400, the SEPIC converter 500 is a DC-to-DC converter. Unlike the Ćuk converter 400, the SEPIC converter 500 is non-inverting. Its output transfer function is defined by             V      OUT        =                  V        IN            xc3x97                        T          ON                          T          OFF                      ,
where TON and TOFF are the on and off times of switching element 506.
The actual construction of the SEPIC converter 500 is also very similar to the Ćuk converter, utilizing a DC input voltage source 502 coupled through an input inductor 504 across switching element 506 and across inductor 508. Unlike the Ćuk converter 400, the SEPIC converter 500 utilizes an output diode 512, coupling inductor 510 between capacitor 508 and diode 512 to ground. The output capacitor 514 is coupled in parallel with the output load 516. While the input current from the positive DC voltage source 502 may be continuous, the structure of the SEPIC converter 500 results in an output current that is discontinuous. This discontinuity in the output current tends to increase the output distortion, and further limits application of the SEPIC converter to applications that can tolerate such increased output waveform distortion. This SEPIC converter 500 is also limited to applications that have only a positive DC input voltage, therefore prohibiting its application where AC line voltage must be used.
In view of the above, it is an object of the present invention to provide a new and improved single-stage power converter. More particularly, it is an object of the present invention to provide a new and improved single-stage switched mode power converter providing power factor correction.
In one embodiment of the present invention a single-stage power converter comprises an input that receives an electric power input from an external source, a first circuit portion coupled to the input and operative during a first phase to produce an output power to an external coupled load. The converter further includes a second circuit portion coupled to the input and operative during a second phase to produce the output power. In this embodiment, the first circuit portion is configured as either a Ćuk converter or a single-ended primary inductance converter (SEPIC) and the second circuit portion is configured as either a Ćuk converter or a single-ended primary inductance converter (SEPIC). Unlike a typical SEPIC converter, the output power produced by each of the first and the second circuit portions is continuous.
In an embodiment of the present invention, the first circuit portion includes an output inductor through which the output power is supplied during the first phase. The second circuit portion shares this output inductor. For an embodiment where the input electric power is AC and the output electric power is DC, the first circuit portion is configured as a Ćuk converter and the second portion is configured as a SEPIC converter. In this embodiment the first circuit portion is operative during a negative half-cycle of the AC input electric power, and the second circuit portion is operative during a positive half-cycle of the AC input electric power.
For an embodiment where the input electric power is AC and the output electric power is also AC, the first circuit portion is configured as a Ćuk converter and the second circuit portion is also configured as a Ćuk converter. The first circuit portion is operative during a positive half-cycle of the AC input electric power, and the second circuit portion is operative during a negative half-cycle of the AC input electric power. With this embodiment the output electric power is inverted relative to the input electric power.
For an embodiment where the input electric power is AC and the output electric power is AC, the first circuit portion is configured as a SEPIC converter and the second circuit portion is also configured as a SEPIC converter. In this embodiment the first circuit portion is operative during a positive half-cycle of the AC input electric power, and the second circuit portion is operative during a negative half-cycle of the AC input electric power. The output electric power is non-inverted relative to the input electric power.
For and embodiment where the input electric power is DC and the output electric power is AC, the first circuit portion is configured as a Ćuk converter and the second circuit portion is configured as a SEPIC converter. In this embodiment the first circuit portion is operative to construct a negative half-cycle of the AC output electric power, and the second circuit portion is operative to construct a positive half-cycle of the AC output electric power.
In a further embodiment of the present invention, the converter further comprises a third circuit portion coupled to the input and operative during a third phase of operation to produce the output power. The third circuit portion is configured as one of a Ćuk converter and a single-ended primary inductance converter (SEPIC) sharing the output inductor through which the output power is supplied. In this embodiment, at least one of the first, second, and third circuit portions is configured as a Ćuk converter and at least one other of the first, the second, and the third circuit portions is configured as a SEPIC converter.
Preferably in this embodiment the input is adapted to selectively receive an electric power input from a first external source of DC electric power and a second external source of AC electric power. When the input receives the electric power input form the first external source of DC electric power and when the output electric power is AC, the first circuit portion is configured as a Ćuk converter and the second circuit portion is configured as a SEPIC converter. In this configuration the first circuit portion is operative to construct a negative half-cycle of the AC output electric power. The second circuit portion is operative to construct a positive half-cycle of the AC output electric power.
In this embodiment when the input receives the electric power input form the second external source of AC electric power and when the output electric power is AC, the second circuit portion is configured as a SEPIC converter and the third circuit portion is also configured as a SEPIC converter. In this configuration, the second circuit portion is operative to construct a positive half-cycle of the AC output electric power, and the third circuit portion is operative to construct a negative half-cycle of the AC output electric power.
In an alternate embodiment a single-stage AC-to-AC converter comprises an input adapted to receive AC electric power from an external source and a first circuit portion. This first circuit portion forms a first Ćuk converter having an input inductor, an output inductor, and a line capacitor. The AC-to-AC converter also includes a second circuit portion forming, in conjunction with the input inductor, the output inductor, and the line capacitor, a second Ćuk converter. This second Ćuk converter is oriented in opposite polarity to the first Ćuk converter. Preferably, the first Ćuk converter is operative during a positive half-cycle of the AC electric power from the external source to generate a negative half-cycle of an output AC electric power. The second Ćuk converter is operative during a negative half-cycle of the AC electric power from the external source to generate a positive half-cycle of the output AC electric power.
In one embodiment, the first Ćuk converter includes a first power switching device and a first series connected diode. Likewise, the second Ćuk converter includes a second power switching device and a second series connected diode. The first power switching device and the first series connected diode are coupled in opposite parallel orientation to the second power switching device and the second series connected diode.
In a further embodiment, a single-stage AC-to-DC converter is presented comprising a first circuit portion forming a Ćuk converter having an input inductor and an output inductor, and a second circuit portion forming, in conjunction with the input inductor of the Ćuk converter, a single-ended primary inductance converter (SEPIC). The SEPIC converter is coupled to the output inductor of the Ćuk converter to maintain a constant output current during operation of the SEPIC converter. Further, the SEPIC converter is oriented in opposite polarity to the Ćuk converter.
Preferably, reverse power flow through the first circuit portion and the second circuit portion is prohibited by blocking diodes. Additionally, in one embodiment the first circuit portion is operative during a negative half-cycle of the AC electric power to produce a positive DC output, and the second circuit portion is operative during a positive half-cycle of the AC electric power to produce a positive DC output. The shared output inductor ensures constant output current during operation of the second circuit portion.
In a further embodiment of the present invention, an uninterruptible power supply (UPS) operative to supply AC output power to connected loads from both AC line power and backup DC battery power in the event of loss or severe degradation of AC line power is presented. This UPS comprises input source selection circuitry that is adapted to receive the AC line power and the backup DC battery power. The UPS also includes a first circuit portion forming a Ćuk converter having an input inductor and an output inductor. A second circuit portion is included that forms, in conjunction with the input inductor of the Ćuk converter, a first single-ended primary inductance converter (SEPIC). The first SEPIC converter is also coupled to the output inductor of the Ćuk converter to maintain a constant output current during operation of the first SEPIC converter. This first SEPIC converter is oriented in opposite polarity to the Ćuk converter. A third circuit portion is also included. This third circuit portion forms, in conjunction with the input inductor of the Ćuk converter, a second single-ended primary inductance converter (SEPIC). The second SEPIC converter is also coupled to the output inductor of the Ćuk converter to maintain a constant output current during operation. This second SEPIC converter is oriented in like polarity to the Ćuk converter.
In this embodiment when the input source selection circuitry selects the AC line power, the second SEPIC converter is operative during a positive half-cycle of the AC line power to produce a positive half-cycle of the AC output power through the shared output inductor of the Ćuk converter. The first SEPIC converter is then operative during a negative half-cycle of the AC line power to produce a negative half-cycle of the AC output power through the shared output inductor of the Ćuk converter. When the input source selection circuitry selects the backup DC battery power, the second SEPIC converter is operative to produce a positive half-cycle of the AC output power through the shared output inductor of the Ćuk converter. The Ćuk converter is then operative to produce a negative half-cycle of the AC output power.
Other objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.